Explosion welding is a known per se technique for attaching together metal objects, such as plates or pipes. In short, two objects are placed on each other, and on top of that a layer of explosive material is arranged. When the explosion material is made to explode, the two objects are pressed together powerfully to such extent that they melt together in the attachment zone. A large advantage with respect to other welding techniques is that the heat input is relatively low, so that the objects as a whole do not melt and no or only a very thin mixing zone results.
A special application of explosion welding is attaching together two concentric pipes within each other, wherein the outer pipe forms a coating for the inner pipe. In such case, the explosive material is arranged around the outer pipe, and the explosion front moves along the length of the pipes. The explosion can be used to clamp the outer pipe against the inner pipe, but a metallurgical joint will result if the explosion is controlled well. In a particular embodiment, the inner pipe is made of inexpensive carbon steel and the outer pipe is made of a more expensive stainless type of material such as for instance inconel 625.
In order to have the circular pipes maintain their shape well, it is necessary that the interior of the inner pipe is filled with a medium that can resist the shock of the explosion well, and a liquid is used for this purpose because of the fact that a liquid is non-compressible. This may be a cheap liquid such as water. This process is well known on a small laboratory scale; the maximum achievable length in this case is about 50 cm. However, there is a need to apply this process in pipes having commercial lengths of 3 m or 6 m, and possibly even larger. In such case, it appears that a problem occurs in practice, of which the precise cause is not entirely clear to us, but which seems to be connected to the propagation of the shock wave in the water. The problem expresses itself in that the work piece exhibits a trumpet-like deformation: the diameter of the work piece increases as the distance to the end of the work piece decreases, i.e. seen in the direction of propagation the explosion front, and it is even possible that the work piece tears open at its end.
The invention aims to provide a solution to this problem.